Proceedings of SPIE, the International Society for Optical Engineering vol:6192 pages:61922A.1-61922A.8
Organic optoelectronics and photonics II location:Strasbourg, France date:3-6 April, 2006
We present the systematic study of charge carrier mobility in 4,4'-N,N'-dicarbazole-biphenyl (CBP) films doped with a triplet-emitter material tris(2-phenylpyridine) iridium (Ir(ppy)3)- 1 wt% and 10 wt%. The hole mobility was investigated with the time-of-flight (TOF) technique, as a function of electric field in the range 105 - 2 x 106 V/cm. For a CBP film doped with 1% of Ir(ppy)3 the hole mobility was also measured as a function of temperature from 20 °C to 70 °C. The obtained hole mobilities in CBP film doped with 1% of Ir(ppy)3 are in the range 5 x 10-10 - 3 x 10-7 cm2/Vs. The measured dependence of the hole mobility on temperature and electric field is explained in the framework of the Gaussian disorder model of Bässler. Calculated values for the effective energetic and the positional disorder are 162 meV and 4.2, respectively. The activation energy at zero field is 0.58 eV. The mobility extrapolated to zero field and infinite temperature amounts to 6 x 10-4 cm2/Vs. In CBP film doped with 10% of Ir(ppy)3 the hole mobilities are in the range 2 x 10-8 - 3 x 10-7 cm2/Vs. The observed increase of the hole mobility in Ir(ppy)3-doped CBP films with increasing doping concentration can be attributed to hopping transport of charge carries via dopant molecules.